Cancer prevention is the most effective strategy to reduce cancer morbidity and mortality. Therefore, it is imperative to develop new investigators in this area of research. Yan Dong, a junior investigator in the field of cancer chemoprevention, has extensive training in molecular biology and cell biology. The support from this award will foster Dr. Dong's development as an independent researcher in the field of human cancer prevention. This project will focus on the investigation of molecular mechanism of selenium chemoprevention of prostate cancer. A previous randomized, placebo-controlled cancer prevention trial showed that selenium supplementation significantly reduced the incidence of prostate cancer, although the mechanism of action is still unknown. The proposed research will test the hypothesis that selenium prevents prostate cancer by suppressing androgen receptor (AR) signaling. The research plan consists of four aims. Aim 1 is to study the mechanism of reduced AR transcription by selenium using the reporter gene assay, mutagenesis analysis, and EMSA. Aim 2 is to investigate selenium-modulation of AR ligand binding, dimerization, and nuclear translocation by the whole-cell radioligand binding assay, protein-protein interaction analysis with a mammalian two-hybrid system, as well as Western and immunofluorescence analyses, respectively. Aim 3 is to assess the functional significance of AR downregulation in selenium chemoprevention of prostate cancer. This issue will be addressed both in vitro and in vivo using stable AR transfectants. Aim 4 is to characterize AR-mediated gene expression changes critical for selenium anticancer effect by the real-time RT-PCR analysis in both in vitro and in vivo models. The overall goal of this project is to sharpen Dr. Dong's scientific and technical acumen essential for carrying out translation research and obtain R01 support for the establishment of an independent research program directly relevant to the prevention of human cancer.